Personally, I'd go with the Shimanos. I used to have the 775's, and can tell from experience, the XT 785's are indescribably better than the 775's. And I really, really liked the 775's. If you like one finger braking, the 785's are THE brakes to have.

As for the XX's, I've heard horror stories about all kinds of bleed problems, brake shudder and levers suddenly pulling to the bar, which is the main reason I'd choose the Shimanos over the XX's. That being said, the brakes are awesomely light and the carbon bling is, well, bling. I also really like the lever geometry.

Get the XT's! They're awesome. I've been on them for a month and still need to be careful grabbing too much brake at one time. Easy setup, easy to shorten the hoses, and you don't need to bleed them out of the box. I haven't tried the newer Avid's, but the Juicy's blow chunks.

A friend of mine almost killed himself with the XX's on his S-works SJ-HT. Even after bleeding and adjusting till they "felt" perfect, they had marginal power. A quick swap to 988's (weight weenie version) and Voila!! Great brakes.